It is commonly known that metals, both in the liquid and the solid state, when cooled undergo a reduction in volume, a so-called thermal contraction. In casting moulds, in which a non-uniform heat distribution reigns in the mould cavity after the pouring, and in which for this reason all parts of the casting do not solidify at the same time, this causes the parts of the casting solidifying last to give off liquid metal to compensate for the contraction of the parts of the casting having solidified earlier, leading to faults in the casting, commonly called "shrinkage holes" appearing in the form of depressions in the surface of the casting or cavities (macroscopic or microscopic holes) within the casting. In order to avoid these casting faults, the skilled person can have recourse to a series of expedients, of which the most common is the use of feeding reservoirs, i.e. cavities in the mould being filled with metal during the pouring and having such dimensions that the metal in them solidifies later than the parts of the casting solidifying last, being connected to the latter through ducts having a relatively large cross-sectional area, thus being able to post-feed these parts with liquid metal to compangate for the contraction. U.S. Pat. No. 1,410,775 describas an example of this method.
In the method described in U.S. Pat. No. 1,410,775, the full flow of the melt passes through the feeding reservoir. As the latter will necessarily have a considerable volume in order to be able to accommodate all the melt required for feeding, this means that the melt will flow through the feeding reservoir in a turbulent manner. If the melt is of an easily oxidizable material, such as aluminium or maqnesium or their alloys, such an arrangement entails the risk of relatively large quantities of oxide being formed, causing a loss of metal and possibly contamination of the castings.